Turbines are used for generating rotary mechanical power from the energy in a working fluid. The working fluid energy, originally in the form of pressure energy, is converted to velocity energy by passing through a system of blades in the turbine. Changes in the magnitude and direction of the velocity energy are made to cause tangential forces on the blades, producing mechanical rotation of a turbine rotor. The rotating turbine rotor may be positioned to interact with a generator rotor and generator stator and thereby produce electricity.
FIG. 1 is a simplified illustration of a rotor assembly 11 that includes a rotor shaft 12 supporting a number of blade assemblies 13, 14, 15, and 16. Each blade assembly includes a blade hub 17 that supports a number of blades.
Turbine rotors can be massive steel components sometimes weighing in excess of 200 tons. Steam turbines are used to convert thermal energy into usable work and are typically used to drive an A-C electric generator to produce electric power. The design of a turbine rotor permits most of the associated hardware to be integral to the rotor itself. The major exception is the blading that protrudes from the outside surfaces of the rotor forging. Blades are designed to take advantage of the steam conditions that change as the steam passes through the different stages of the turbine. The first blades encountered at the steam inlet are small in size but capable of withstanding high temperatures and pressures. As seen in FIG. 1, successive stages of the blades become larger to take advantage of the expanding steam as it cools. The blades are individually machined and assembled onto the rotors in groups.
The attachment of the blades to the rotor is accomplished by having machined surfaces on the blade and on the rotor that work together in a locking mechanism. The design of the blade attachment area varies among manufacturers and may vary from row to row on an individual rotor. In addition to the primary function of holding the blade to the rotor, the attachment area is designed to make it relatively easy to change the blades if they should be damaged during service. Even so, blade removal can be extremely time consuming. The process also has the potential for damaging the blades beyond repair. Consequently, blade removal is best avoided when possible.
There are several designs for the region where the turbine blades attach to the rotor. One of the more common attachment mechanisms is the straddle-mount design, FIG. 2. In this design, the hub 17 is carried on the rotor shaft 12. An arrangement of "hooks" 21 is machined around the entire periphery of the hub or "disk" in a fir tree configuration. The hub can either be an integral part of the rotor forging, machined to a disk shape, or a separate disk assembled to the central shaft. The blades include an attachment configuration that matches and engages the hooks 21 of the hub rim. At some location around the rim of the disk, the hooks are removed, i.e., machined away, for the width of one blade to provide a loading slot 22. Blades are loaded onto the disk rim by first mounting a blade over the rim at this loading slot and then moving it around the disk to engage the mating hooks. This is done sequentially, one blade at a time, until the entire periphery has been loaded. A special "closing" blade (not shown) is used in the loading slot. This blade is attached either by drilling and pinning it to the rim, by locking it to adjacent blades on either side of it, or by a combination of the two. The "hook" area of the attachment is under extremely high centrifugal stress when the rotor is rotating, making it susceptible to service-induced flaws or cracks 23. Although service-induced flaws are not restricted to the straddle-mount design, for the purposes of the disclosure, only the straddle-mount design is described.
Thermal and centrifugal stresses constantly act on the attachment areas when a turbine is in service. In certain areas where wet steam is present, possible aided by caustic steam conditions arising from less than adequate control of steam chemistry, and where stresses are adequate, service-induced flaws may initiate and grow by an intergranular stress corrosion cracking (IGSCC) mechanism in the attachment region. High temperature creep, mechanical fatigue, and combinations of the mechanisms may also cause cracking or aid the initiation and growth of stress corrosion cracking. Service-induced flaws on the blade side of the attachment occur occasionally. Generally, blade cracking occurs less frequently than disk cracking because less susceptible material can be used for the blades. In addition, blades tend to fail individually, resulting in limited damage. For service-induced flaws on the hub side of the attachment, however, initiation leading to propagation is not restricted to the region under only one blade. Left unchecked, cracking in the hub side of the attachment may, in certain designs, propagate under successive blades and groups of blades until catastrophic failure of the affected area occurs. Damage to the turbine and surrounding components outside the turbine may occur when large pieces of the rotor cannot be contained by the turbine shells.
Ultrasonic testing procedures are commonly used to examine turbine components for the purpose of detecting and characterizing service-induced flaws. The technique involves applying high frequency sound waves to a structure of interest. When the sound waves interact with an object that has a significant difference in acoustic impedance (the product of density and acoustic velocity) from that of the propagation medium, a portion of the sound is either reflected or diffracted back to the source from which the sound originated. Measurement and evaluation of the returned sound pattern permits determination of the presence and characteristics of the reflecting medium.
For ultrasonic techniques to work, it is necessary to discriminate between object architecture and flaws in the object architecture. This discrimination is readily accomplished when the object architecture is known. The test for service-induced flaws on the disk side of the blade attachment required knowledge of the attachment interlocking geometry. Several blade attachments are designed such that the surface geometry is visible from the side of the disk. However, the straddle-mount design is not visible from the side of the rotor.
Ideally, inspection of the hub blade attachment is best accomplished with the aid of machine drawings of the attachment region. Rotor manufacturers have machine drawings, but because of their proprietary nature, they are reluctant to provide them to outside sources for the purpose of inspection. Without the machine drawings, or knowledge of the attachment geometry, attempting to perform the geometry-dependent nondestructive evaluation is less than reliable.
Typically, the straddle-mount blade attachment region has been inspected by mocking up a scale drawing of its cross sectional area. The mockup will provide the specific transducer angles and locations necessary for the piezoelectric transducer to direct the sound at the points where service-induced flaws should initiate. This also provides the means to evaluate various reflectors to determine which are from the geometry of the blade attachments themselves, and which are from flaws emanating from the geometry. Essentially, this limitation reduces the effectiveness of anyone attempting to perform the inspection that had to rely on another method of determining the geometry without the blade attachment drawings. The process for performing the inspection then calls for the transducer to be placed on the side of the disk and, with the appropriate angle, aimed at the "hook" of interest on the opposite side of the disk. Scanning around the circumference of the hub is to be performed by using a fixture to place the transducer on the radial face disk and rotating the rotor while maintaining the relative transducer position dictated by the mockup drawing. Adjacent hooks are inspected by repositioning the transducer along the same radial face and again rotating the rotor. One scan is required for each hook to be inspected. Data obtained during the inspection can either be recorded manually or by the use of an automated system.
In U.S. Pat. No. 5,408,884 there is described a method for ultrasonic reconstruction of the turbine blade attachment structure. After the structure has been reconstructed, the inspection is then carried out as described above.